Ion implanters are commonly used in the production of semiconductor wafers. An ion source is used to create an ion beam, which is then directed toward the wafer. As the ions strike the wafer, they dope a particular region of the wafer. The configuration of doped regions defines their functionality, and through the use of conductive interconnects, these wafers can be transformed into complex circuits.
A block diagram of a representative ion implanter 100 is shown in FIG. 1. Power supply 101 supplies the required energy to the ion source 102 to enable the generation of ions. An ion source 102 generates ions of a desired species. In some embodiments, these species are mono-atoms, which are best suited for high-energy implant applications. In other embodiments, these species are molecules, which are better suited for low-energy implant applications. The ion source 102 has an aperture through which ions can pass. These ions are attracted to and through the aperture by electrodes 104. These exiting ions are formed into a beam 10, which then passes through a mass analyzer 106. The mass analyzer, having a resolving aperture, is used to remove unwanted components from the ion beam, resulting in an ion beam having the desired energy and mass characteristics passing through resolving aperture. Ions of the desired species then pass through a deceleration stage 108, which may include one or more electrodes. The output of the deceleration stage is a diverging ion beam.
A corrector, or collimator, magnet 110 is adapted to deflect the divergent ion beam into a set of beamlets having substantially parallel trajectories. Preferably, the collimator magnet 110 comprises a magnet coil and magnetic pole pieces that are spaced apart to form a gap, through which the ion beamlets pass. The coil is energized so as to create a magnetic field within the gap, which deflects the ion beamlets in accordance with the strength and direction of the applied magnetic field. The magnetic field is adjusted by varying the current through the magnet coil. Alternatively, other structures, such as parallelizing lenses, can also be utilized to perform this function.
Following the angle corrector 110, the ribbon beam is targeted toward the workpiece. In some embodiments, a second deceleration stage 112 may be added. The workpiece is attached to a workpiece support 114. The workpiece support 114 provides a variety of degrees of movement for various implant applications.
The components between the ion source and the workpiece comprise the beamline. These components transform the ions generated by the ion source into a ribbon beam capable of implanting ions into a workpiece, such as a semiconductor wafer.
The workpiece support is used to both hold the wafer in position, and to orient the wafer so as to be properly implanted by the ion beam. To effectively hold the wafer in place, most workpiece supports typically use a circular surface on which the workpiece rests, known as a platen. Often, the platen uses electrostatic force to hold the workpiece in position. By creating a strong electrostatic force on the platen, also known as the electrostatic chuck, the workpiece or wafer can be held in place without any mechanical fastening devices. This minimizes contamination and also improves cycle time, since the wafer does not need to be unfastened after it has been implanted. These chucks typically use one of two types of force to hold the wafer in place: coulombic or Johnson-Rahbeck force.
The workpiece support typically is capable of moving the workpiece in one or more directions. For example, in ion implantation, the ion beam is typically a scanned or ribbon beam, having a width much greater than its height. Assume that the width of the beam is defined as the x axis, the height of the beam is defined as the y axis, and the path of travel of the beam is defined as the z axis. The width of the beam is typically wider than the workpiece, such that the workpiece does not have to be moved in the x direction. However, it is common to move the workpiece along the y axis to expose the entire workpiece to the beam.
The uniformity of the ion beam is critical to successful implantation. Low energy levels, such as those used for shallow implants, make it even more difficult to maintain ion beam uniformity. If the concentration of ions across the beam width is uneven, the implanted substrate will have non-uniform characteristics and parameters, which will affect its performance and usefulness.